1. Field of the Invention
This invention relates to the thermal treatment of glass and more especially to the thermal toughening of flat glass or bent glass sheets, for example glass sheets for use singly as a motor vehicle windscreen, or as part of a laminated motor vehicle windscreen, or a side light or rear light for a motor vehicle, or for use in the construction of windscreen assemblies for aircraft and railway locomotives.
2. Description of the Prior Art
In our U.S. Pat. No. 4,113,458, there is described a method for thermally treating glass articles, by heating each glass article to a temperature above its strain point, and quenching the glass articles in turn in a gas-fluidised bed of particulate material which is placed in a quiescent uniformly expanded state of particulate fluidisation by control of the distribution of fluidising gas in the particulate material at a gas flow velocity through the particulate material between that velocity corresponding to incipient fluidisation and that velocity corresponding to maximum expansion of the particulate material.
This state of fluidisation of the bed is such that agitation of the fluidised particulate material is engendered on the hot immersed surfaces of the glass as the glass cools in the fluidised bed, but any transient tensile stresses induced in the surface of the hot glass as its leading edge first contacts the fluidised bed are not so severe as to endanger the glass. Therefore the process has a high yield.
The degree of toughening of a glass sheet which is immersed in such a fluidised bed depends on the rate of heat transfer between the fluidised particulate material and the hot sheet immersed in it, and on the rapid transfer of hot particles away from the vicinity of the glass sheet with a concurrent rapid supply of cooler particles from the body of the fluidised bed into the vicinity of the glass sheet.
The movement of particles in the vicinity of the glass surfaces is more rapid than the movement of the particles in the bulk of the bed, because of rapid agitation of the fluidised particulate material which is engendered on the hot immersed surfaces of the glass due to heating of the particulate material by the glass which continues as the glass cools in the fluidised bed.
As described in our co-pending U.S. patent application Ser. No. 934,728, entitled "Thermal Toughening of Glass", agitation of the particulate material at the glass surfaces is considerably enhanced when using a selected particulate material which has latent gas-evolution properties such that there is a rapid evolution of gas from the particulate material when heated in proximity to the glass surfaces.
It has now been found that there are three factors which dominate in controlling the thermal toughening of glass in a gas-fluidised particulate material, and in particular which control the degree of toughening of a hot glass sheet when contacted with a gas-fluidised particulate material.
These factors are as follows:
1. The gas-generating properties of the particulate material.
2. The thermal capacity per unit volume of the particulate material at minimum fluidisation. This is derived from the specific heat of the material measured at 50.degree. C. and the density of the material of the bed measured at minimum fluidisation of the material.
3. The "flowability" of the particulate material, as defined below, which is the sum of four point scores which are awarded to the material by assessment of four characteristics of the flowable particulate material. The term "flowability" when used herein has that meaning.
These four characteristics of a flowable particulate material and the manner of awarding point scores are described in the article "Evaluating Flow Properties of Solids" by Ralph L. Carr Jr., Chemical Engineering Volume 72, Number 2, Jan. 18, 1965, and are as follows:
1. Compressibility=100 (P-A)/P% where
P=packed bulk density and PA1 A=aerated bulk density
2. Angle of Repose: this is the angle in degrees between the horizontal and the slope of a heap of the particulate material dropped from a point above the horizontal until a constant angle is measured.
3. Angle of Spatula: a spatula is inserted horizontally into the bottom of a mass of the dry particulate material and is lifted straight up and out of the material. An average value of the angle in degrees to the horizontal of the side of the heap of material on the spatula is the Angle of Spatula.
4. Particle Size Distribution (called Uniformity Co-efficient in the above mentioned article): this is described in the above mentioned article as the numerical value arrived at by dividing the width of sieve opening (i.e. particle size) which will pass 60% of the particulate material by the width of sieve opening which will just pass 10% of the particulate material.
All the values of particle size distribution referred to herein were measured in known manner by a method using a Coulter counter to determine the particle diameters appropriate to retained cumulative weight percentages of 40% and 90% corresponding to widths of sieve openings which will pass 60% and will just pass 10% of the particulate material.
The numerical values of Compressibility, Angle of Repose, and Angle of Spatula were measured using a Hosakawa Powder Tester manufactured by the Hosakawa Micrometrics Laboratory, of The Hosakawa Iron Works, Osaka, Japan, which Powder Tester is specifically designed for use in the determination of the "flowability" of powders as defined above.
It will be appreciated that the flowability of a particulate material is basically related to factors such as the mean particle size, the particle size distribution, and the shape of the particles which is sometimes referred to as the angularity of the particles, that is whether they have a rounded or angular shape. The value of flowability increases with increase of the mean particle size, with decrease of the particle size distribution, and with decrease in the angularity of the particles.
It will also be appreciated that the thermal capacity per unit volume at minimum fluidisation is dependent on the specific heat of the material and on the density of the fluidised bed at minimum fluidisation, which density increases with decrease of the particle size distribution.
A high value of toughening stress is produced in glass when it is quenched in a fluidised bed having an optimum flowability. Some materials which produce required toughening stresses may be directly selected from those which are commercially available. Other commercially available materials may be modified to produce the required toughening stresses by sieving the material to change its mean particle size and particle size distribution.
A problem exists however in that materials having the required flowability may not be commercially available, and there is a limit to the extent to which the degree of toughening stress induced in glass can be controlled by variation of the flowability of commercially available materials. The production of a large quantity of a material having the required flowability may involve the sieving of a large quantity of particulate material. In addition when using a single material the modification of the thermal capacity of the fluidised bed is accomplished by narrowing the particle size distribution. This modification of the thermal capacity in turn affects the variation of flowability which is produced by narrowing the particle size distribution.
It has now been found that a particulate material can be produced having optimum gas-generating properties, thermal capacity and flowability for the production of required toughening stresses in a glass article by use of a mixture of particulate materials each of which contributes to optimum properties of the mixture. By selection of particulate materials and the proportions in which they are mixed the gas-fluidised particulate material can be tailored to provide any required toughening stresses within a wide range.